Purification and adjustment of electrolytes



United States Patent PURIFICATION AN D ADJUSTMENT 0F ELECTROLYTES Myron Ceresa, Pittsburgh, and Wenzel L. Bohman, Port 'Vue, Pa., assignors to Westinghouse Electric Corporation, 'East Pittsburgh, Pa., a corporation of Pennsylvania No'Drawing. Application December 23, 1954 Serial No. 477,424

7 Claims. (Cl. 204-52) The present invention relates to the treatment of electrolytes and particularly alkali cyanide electrolytes in order to purify them and to adust the components for better electroplatingand the like.

One of the major problems arising in the operation of-a'lkali cyanide electroplating electrolytes is the tendency for the composition to become unbalancedso that it no longer. produces'satisfactory electroplating and the speed of metal deposition is. materially reduced. In particular, the carbonates tend to build up rapidly to a great excess which may amount to from 20 to 25 ounces of alkali metal carbonate per gallon of electrolyte, and this is far above the desired proportions which usually are from 4 to 8 ounces per gallon, and preferably not to ex ceed 10 ounces per gallon to produce the best plating.

Buildup of the carbonate content in alkali cyanide electrolytes has been particularly severe in plating installations wherein the rinse Water applied to the plated members to wash off the dragout cyanide solution is returned to the plating tank. Owing to strict health and sanitation requirements, cyanides cannot be discarded to the drain. However, the rinse waters pick up a substantial amount of carbon dioxide from the air and this is added to the plating tank under present-day practices. Previously it was a common practice to discard to .a drain the rinse waters containing the electrolyte dragout. In normal operation of a conveyorized plating line, the dragout losses were sufiicient to equal a complete turnover of the electroplating electrolyte every 3 to 6 months on the average. Such dragout tended to keep the carbonate content of a cyanide electrolyte at a low level andconsequently excessive carbonates did not constitute any major difficulty. The return of the rinse waters to the electroplating tank, however, aggravates the carbonate buildup probleml It is well known to those in the electroplating industry that organic impurities tend to accumulate in plating tanks. Such organic impurities have been found in many cases to affect .thequality of the electrodeposited metal. Such organic contaminants may comprise material leached out of the tank lining and rack coatings. Contaminated air bearing oil and organic dust particles contribute a substantial amount of organic impurities, particularly where air agitation is employed; In some cases organic matter may be added inadvertently or in some casesit may be the result of the decomposition of organic addition agents in the electrolyte.

In many cases, the work being plated in the electrolyte drags in some amounts of organic and other impurities such as chromates and sulfates. The chromates and sulfates are generally undesirable components" and should be kept at a very low concentration in the alkali cyanide plating bath.

The following react-ionshave been found to occur and result in depletion of certain constituents of the plating bathpwhile simultaneously increasing the carbonate content.

2. (1) ZNaCN 00: Hi0 NaiCOri-QHCNT (2) 2NaOH 00; INBJCO: 11,0

The above equations relates-to the reactions of sodium compounds but similar reactions apply. to the potassium compounds. It will be observed that in Equation 1 sodium cyanide is depleted, while in Equationl sodium hydroxide is depleted, InEquations 3 and 4, sodium cyanide. and sodium hydroxide are destroyed by :the carbonate forming reaction It is well known that there is a specified range of proportions within which the eomponents of an electrolyte mustbe maintained; in order to produce the best plating results. If anyof the components of an alkaline cyanide electrolyte such as the alkali; metal hydroxide, alkali metal cyanide or alkali :metal carbonate, depart from specification, then the plating operation will be less satisfactoryand the quality and speed of plating will invariably suffer in proportion to such departure from the desired proportions. Inasmuch as most of the car-bonate forming reactions deplete the alkali metal cyanide and alkali metal hydroxide, there not only results an undesirable excess of carbonate but also. a reduction of the free alkali. metal cyanide present in the electrolyte and an undesirablechange in the alkalinity of the. electrolyte.

While it has beenproposed in the art to add some one ingredient, such as gypsum (calcium sultate),magnesium carbonate, barium hydroxide, or calcium hydroxide to an electrolyte in order to precipitate magnesium, barium or calcium carbonates therefrom, the, treated electrolyte is not in proper balance or adjustment with respect to free cyanide or the hydroxidecontent; In some .cases the addition of such carbonate removing agentshas required a plurality of succeeding adjusting treatments of the electrolyte in order to bring it intoreasonable operating balance.

The addition of calcium Jhydroxidealoneto a plating electrolyte hasibeen found to result in a number of unsatisfactory features. In many. cases, the calcium carbonate is in the form of a colloidal or slimy precipitate that settles out of theelectrolyteextremely slowly and it is difiicult to filter such slimy precipitate.

The objectof this invention is to provide a process for treating alkali cyanideelectrolytes with a single composition in order to remove excessive carbonates and to put the electrolyte within the desired operating specification.

A further object of the invention is to provide for treating aqueous alkaline cyanide electrolytes with a composition comprising calcium cyanide, alkali metal cyanide and calcium oxide in such proportions that the excessive carbonates are precipitated and the electrolyte is returned to specification.

A still further object is to provide a composite treating material which, when applied to aqueous alkaline cyanide electrolytes, will purify the electrolyte and produce a coarse rapid settling carbonateprecipitate.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

We have discovered that an aqueous alkali cyanide electroplating electrolyte out of desired operating specifications by reason of an excess of carbonate, a deficiency of free alkali metal cyanide and containing impurities, may be rapidly and effectively treated to put itback into operating specification by adding to it. anadmixture of calcium cyanide, alkali metal cyanide and calcium oxide so proportioned and in such amount that the excess carbonate forms a coarse rapid settling precipitate of calcium carbonate which occludes organic impurities and the free alkali metal cyanide content is increased and the alkalinity or pH of the solution is maintained at a level imparted by not over substantially 10 ounces per gallon of alkali metal hydroxide. The composition is agitated vigorously with the electrolyte for a period of time until the reaction is substantially complete, then the agitation is terminated and the solution permitted to stand to allow the precipiate to settle out. Thereafter the treated elec trolyte can be readily separated from the precipitate. Thus, in a single brief treatment an electrolyte that is out of specification due to carbonate buildup and impurity contamination may be purified and put back into specification.

The composition applied to the aqueous alkali cyanide electrolyte preferably comprises as its essential ingredients calcium cyanide and alkali metal cyanide and calcium oxide in the proportions of from 25% to 75% by weight of calcium cyanide, from 45% to 10% by weight of one or more alkali metal cyanides and from 50% to 5% by weight of calcium oxide. As will be pointed out hereinafter, up to 20% of the calcium components may be replaced with barium compounds. Ordinarily the replacement is effected by substituting barium cyanide for the calcium cyanide. It will be understood that calcium hydroxide may replace all or a part of the calcium oxide.

We have secured best results by employing a flaked composition comprising an admixture of these three components. While a chemically pure composition may be employed, we have found that a composition may be manufactured commercially in which these three components are the prime or major ingredients but contain small amounts of the order of a few percent of calcium carbide, calcium cyanamide, and calcium salts. Insoluble materials, such as carbon, metal oxides, halogen salts and small amounts of sulfides also may be present without detriment. In some cases we have dissolved the composition in water to produce a concentrated solution which may be added to the electrolyte. The solid composition may be finely divided, 20 mesh and finer, if desired.

In applying the compositions of this invention to alkali cyanide electrolytes, it is desirable that the electrolyte be at a temperature of not over 140 F., and preferably at or near room temperature, for example, 70 F. to 80 F. If the calcium cyanide compositions are added to electrolytes while at 145 F. and higher, the calcium cyanide tends to form some resinous polymers such as azulmic acid, which will not react with the carbonates in the electrolyte. The removal of these resinous polymers requires the use of excessive amounts of activated carbon.

The following compositions are illustrative of those suitable for practicing the invention,

Example IV Percent Percent Percent Calcium cyanide 55 50 Barium cyanide. 5 5 2 Sodium cyanide 15 18 Potassium cyanide 20 Calcium oxide 25 25 40 The compositions of Examples I to IV may be combined or admixed with up to 20% of their weight with carbon, other calcium salts and inert ingredients.

The following examples are illustrative of the practice of the invention.

Example V A 1000 gallon tank of electrolyte for copper plating had the following composition.

0z./gal. Free KCN 2 K 00 17.6 KOH 3.9 CuCN 6.3

The electrolyte while at F. was treated by sifting into it 480 lbs. of a composition comprising 46.8% of calcium cyanide, 27.3% sodium cyanide and 7.9% calcium oxide, the balance being calcium carbide, carbon, and other salts. After the addition the electrolyte was stirred for an hour, and the precipitate settled out rapidly and separated. The electrolyte composition was as follows.

0z./ gal. Free KCN 10.2 K CO 10 KOH 5 CuCN 6.3

In order to increase the metal content and to reduce the free cyanide, there was added 400 pounds of copper cyanide to the electrolyte. The electrolyte was in better balance for electroplating and produced definitely improved copper plating both in color, brightness and structure.

Example VI weight of sodium cyanide and 10% of calcium oxide, the balance being carbon, small amounts of calcium carbide, calcium cyanamide, calcium fluoride and insolubles. The composition was in the form ofiiakes. The electrolyte was vigorously agitated with a propeller type stirrer and the composition was introduced thereinto over a period of time of approximately one hour.-

The thoroughly stirred electrolyte was then permitted to stand quiescent. A coarse rapidly settling precipitate formed and was permitted to settle out. The clear electrolyte was readily decanted and passed through a filterv to assure that none of the precipitate was present. Ap-

proximately 800 pounds of precipitate was deposited at the bottom of the tank. The precipitate was washed. with deionized water and filtered in order to remove all:

of the adherent soluble cyanide and other components of the electrolyte. The filtrate was added to the treated,

electrolyte. The free cyanide in the electrolyte had increased considerably. The carbonate content was ra assess? duced to an amount beldw resumes-per gallon. The hydroxide content increased b'y' ane ounce p'en ga-llon. The coarse precipiate had occluded the organic impurities almost completely. TheelcttOlY't'Was in specification. 1

Example VII An aqueous copper cyanide electroplating electrolyte containing a carbonate content of above 10 ounces per gallon, with a deficiency of free metal cyanide and having as undesirable impurities therein organic contaminants, chromates and sulfates, is treated by adding thereto one pound per gallon of a composition containing 35% calcium cyanide, 5% barium cyanide, 40% calcium oxide and 20% sodium cyanide. The mixture is agitated for 15 minutes after the addition of the composition and then permitted to stand so that the relatively coarse precipitate that forms will settle out. The precipitate comprises barium chromates and sulfates as well as barium and calcium carbonates. The precipitate oceludes the organic impurities. After treatment, the electrolyte will be within specification with rsepect to the carbonates, free alkali metal cyanide and alkali hydroxide.

It is desirable that the calcium oxide or calcium hydroxide in the composition be present in such amounts that when added to the electrolyte, it will not increase the alkalinity or pH excessively. In general, the best balance is obtained in conventional electroplating electrolytes having excess carbonate by treating them with a composition containing approximately 50% by weight of calcium oxide or calcium hydroxide, or both, the balance being calcium cyanide and the alkali metal cyanide.

The alkali metal cyanide may comprise sodium cyanide, potassium cyanide or lithium cyanide, or mixtures of any two or all three. Thus in treating potassium copper cyanide plating baths, potassium cyanide will be the alkali metal cyanide. We have found that mixed potassium and sodium cyanide are suitable components of the composition for treating copper cyanide electroplating electrolytes.

In some cases, we have secured good results by adding absorption agents such as activated carbon to the electrolyte, either simultaneously with the calcium cyanide compositions, or soon thereafter, and filtered out both the calcium carbonate precipitate and the activated carbon.

The present invention may be applied to alkali cyanide baths employed for plating gold, silver, copper, brass, zinc and cadmium.

It will be understood that the above description is illustrative and not limiting.

We claim as our invention:

1. In the process of treating an aqueous alkali cyanide electrolyte from which may be plated a metal of the group consisting of gold, silver, copper, brass, zinc and cadmium containing an excess of carbonate, having a deficiency of free alkali metal cyanide and having organic impurities, the steps comprising adding to the electrolyte a composition comprising an admixture of calcium cyanide, alkali metal cyanide and calcium oxide in the proportions of from 25 to 75% by weight of calcium cyanide, from 45% to by weight of alkali metal cyanide and from 50% to 5% by weight of calcium oxide, the composition being added in an amount sufficient to react with the excess of the carbonate to form a precipitate of calcium carbonate, there remaining not less than about 4 ounces per gallon of alkali carbonate in the electrolyte, and to introduce additional free alkali metal cyanide to remedy the deficiency, the composition maintaining the pH of the solution at a level of not over substantially 10 Ounces per gallon of alkali hydroxide to promote the rapid settling of the calcium carbonate recipitate man easily filteiable, relatively coarse particle size which precipitate occludes organid'ir'npu'ritiesin the electrolyte, agitatingthe electrolyte and the added composition until the reaction is substantially complete, terminating the agitation andmaintainingthe electrolyte quiescent to allow the precipitate to settle out, and; separating the treated electrolyte fromthe precipitates 2. The process of claim 1, wherein calcium hydroxide replaces at least a part of the calcium oxide.

3. The process of claim 1, wherein the precipitate is washed with water and filtered, the filtrate being added to the previously separated electrolyte.

4. The process of claim 1, wherein a finely divided absorption agent is added to the electrolyte during agitation.

5. In the process of treating an aqueous alkali copper cyanide electrolyte containing in excess of 8 ounces per gallon of alkali carbonate, less than 0.8 ounce per gallon of free alkali metal cyanide, and excessive organic impurities, sulfates and chromates, the steps comprising adding to the electrolyte While it is at a temperature not in excess of F. a composition comprising calcium cyanide, barium cyanide, alkali metal cyanide and calcium oxide, in the proportions of from 25% to 75% by weight of both calcium cyanide and barium cyanide, the barium cyanide not exceeding 20 mole percent of the total calcium and barium cyanides, from 45% to 10% by weight of alkali metal cyanide and from 50% to 5% by weight of calcium oxide, the composition being added in an amount suflicient to provide barium cyanide in an amount reactable with all of the chromate and sulfate to produce a precipitate of barium chromate and barium sulfate, and to react with the excess carbonate to form a calcium carbonate precipitate therewith, there remaining at least about 4 ounces per gallon of alkali carbonate in the electrolyte, and to introduce free alkali metal cyanide to bring the total in the electrolyte to an amount above 0.8 ounce per gallon, the composition maintaining the pH of the solution at a level equal to not over 10 ounces per gallon of alkali hydroxide, to promote rapid settling out of the precipitates in an easily filterable, coarse particle size, which occludes organic impurities, agitating the electrolyte and the added composition until the reactions are substantially complete, terminating agitation and maintaining the electrolyte quiescent to allow the precipitates to settle out, and separating the treated electrolyte from the precipitate.

6. The process of claim 5, wherein calcium hydroxides replace at least a part of the calcium oxide in the composition.

7. In the process of treating an aqueous alkali copper cyanide electroplating electrolyte containing over 8 ounces per gallon of alkali carbonate and less than 0.8 ounce per gallon of free alkali metal cyanide and having organic impurities present, the step comprising adding to the aqueous electrolyte a composition comprising an admixture of calcium cyanide, sodium cyanide and calcium oxide, in the proportion of from 50% to 60% by weight of calcium cyanide, from 40% to 30% by weight of sodium cyanide and from 10% to 20% by weight of calcium oxide, the composition being added in an amount sufiicient to react with the alkali carbonate in excess of 8 ounces per gallon to form a precipitate of calcium carbonate, there remaining at least about 4 ounces per gallon of alkali carbonate in the electrolyte and to introduce alkali metal cyanide to bring the total free alkali cyanide above 0.8 ounce per gallon but not above 6 ounces per gallon, the composition maintaining the alkalinity of the electrolyte at a level not exceeding that produced with 10 ounces per gallon of sodium hydroxide, agitating the electrolyte and added composition for at least 5 minutes, terminating agitation and maintaining the electrolyte quiescent whereby a relatively coarse calcium carbonate precipitate occluding organic impurities rapidly settles out, and sepalzting the treated electrolyte FOREIGN PATENTS from the precipitate. 694,893 Great Britain July 29, 1953 References Cited in the file of this patent OTHER' REFERENCES 7 v v UNITED STATES PATENTS 5 Transactions Electrochem. Soc., vol. 80 (1941), pp. 2,434,191 Banner et a1. -Q. Jan. 6, 1948 358-359.

2,787,590 Rinker Apr. 2, 1957 

1. IN THE PROCESS OF TREATING AN AQUEOUS ALKALI CYANIDE ELETROLYTE FROM WHICH MAY BE PLATED A METAL OF THE GROUP CONSISTING OF GOLD, SILVER, COPPER, BRASS, ZINC AND CADMIUM CONTAINING AN EXCESS OF CARBONATE, HAVING A DEFICIENCY OF FREE ALKALI METAL CYANIDE AND HAVING ORGANIC IMPURTIES, THE STEPS COMPRISING ADDING TO THE ELECTOLYTE A COMPOSITION COMPRISING AN ADMIXTURE OF CALCIUM CYANIDE, ALKALI METAL CYANIDE AND CALCIUM OXIDE IN THE PROPORTIONS OF FROM 25% TO 75% BY WEIGHT OF CALCIUM CYANIDE, FROM 45% TO 10% BY WEIGHT OF ALKALI METAL CYANIDE AND FROM 50% TO 5% BY WEIGHT OF CALCIUM OXIDE, THE COMPOSITION BEING ADDED IN AN AMOUNT SUFFICIENT TO REACT WITH THE EXCESS OF TH CARBONATE TO FORM A PRECITATE OF CALCIUM CARBONATE, THERE REMAINING NOT LESS THAN ABOUT 5 OUNCES PER GALLON OF ALKALI CARBONATE IN THE ELECTROLYTE, AND TO INTRODUCE ADDITIONAL FREEE ALKALI METAL CYANIDE TO REMEDY THE DEFICIENCY, THE COMPOSITION MAINTAINING THE PH OF THE SOLUTION AT A LEVEL OF NOT OVER SUBSTANTIALLY 10 OUNCES PER GALLON OF ALKALI HYDROXIDE TO PROMOTE THE RAPID SETTING OF THE CALCIUM CARBONATE ORECIPATE IN AN EASILY FILTERABLE, RELATIVELY COARSE PARTICLE SIZE WHICH PRECIPITATE OCCULDES ORGANIC IMPURITIES IN THE ELECTROLYTE, AGITATING THE ELECTROLYTE AND THE ADDED COMPOSITION UNTIL THE REACTION IS SUBSTANTIALLY COMPLETE, TEMPERATING THE AGIATION AND MAINTAINING THE ELECTROLYTE QUIESCENT TO ALLOW THE PRECIPITATE TO SETTLE OUT, AND SEPARATING THE TREADED ELECTROYLTE FROM THE PRECIPITATE. 